Polyurethanes are high molecular weight compounds which have a high degree of strength, hardness, and friction resistance. They are often used as adhesives, cements, and coatings. They are made of polymers which contain repeating urethane groups; as shown in FIG. 1.
Traditionally, polyurethanes may be produced by reacting diols with di-isocyanates, as shown in FIG. 3. U.S. Pat. No. 4,401,499 by Kaneko et. al discloses a method for producing a resin which reacts molecules containing hydroxyl groups with di-isocyanates to form temporary urethane polymers, which are noted for their stability and strength. Isocyanates, however, are highly toxic, non-stable substances because they react easily with water, such as moisture in the air. This method of producing polyurethane cannot be used in many applications, namely those involving direct contact with water.
U.S. Pat. No. 5,175,231 by Rappoport, incorporated herein by reference, describes a method for producing water-compatible polyurethanes which involves reacting oligomeric cyclocarbonates with diamines. This method begins with aliphatic polyepoxy molecules, which are used as precursors for cyclocarbonate-containing molecules. An example is Heloxy 84.RTM. produced by Shell Chemical Company. This product contains several epoxy functional groups. Since a desirable polyurethane coating comprises molecules attached to each other in a non-dissipated, three-dimensional network, it is preferable to have a precursor containing as many multi-functional epoxy groups as possible. However, the aliphatic polyepoxy molecules normally contain a number of residual hydroxyl groups that were not converted to epoxy groups, as epoxidation of aliphatic molecules is never 100% efficient due to the nature of the reaction commonly used. As a result, there are fewer epoxy groups available than desired, thus reducing the number of cyclocarbonate groups and possible links in the future polyurethane network. Although it is possible to increase the percentage of epoxy groups, it is more expensive and technically difficult to reach comprehensive epoxidation of aliphatic hydroxyl-containing compounds such as Heloxy 84.RTM..
In Rappoport's method, the epoxy functional groups of Heloxy 84.RTM. are reacted with carbon dioxide to produce cyclocarbonate functional groups. The reaction is shown in FIG. 2. This reaction is also less than 100% efficient, leaving some epoxy groups unreacted. Like the residual hydroxyl groups mentioned above, these unreacted epoxy groups reduce the functionality of the urethane molecule and as a consequence, the number of links in the final polyurethane network. Typically, the conversion rate of epoxy groups to cyclocarbonate groups is only about 80-85% efficient at the soft conditions before "sticking" of the carbonation reaction occurs. In order to achieve comprehensive carbonation, a more extreme version of the reaction must be carried out. The temperature is raised from 100.degree. C. to 130.degree. C., the reaction time is increased from 1-1.5 hours to 5-6 hours, and a larger amount of catalyst, usually quaternary ammonium salts, is used. Though this reaction ensures that nearly all the epoxy groups have been turned into cyclocarbonate groups, it also produces undesirable side reactions and products. In addition, it is more expensive and time-consuming.
After the formation of cyclocarbonate functional groups, the molecules are reacted with diamines, such as Vestamine IPD (isophorone diamine) and Vestamine TMD (trimethyl hexamethylene diamine), both made by Huls America, Inc. The reaction is shown in FIG. 4. These diamines contain two amine groups with different reactivities. For the isophorone diamine, the aliphatic amine groups are the more reactive amines, while the cycloaliphatic amine groups are the less reactive amines. The more reactive aliphatic amines are usually used to react with the cyclocarbonate groups of the molecules, thus forming urethane links. The less reactive cycloaliphatic amines are left unreacted. The urethane links form the basis for the urethane-containing amine hardener. The amine hardener is usually packaged and stored until it is time to create the polyurethane.
In order to create the polyurethane, the urethane-containing molecules of the amine hardener containing the unreacted less reactive cycloaliphatic amines are combined with an epoxy resin. These less reactive cycloaliphatic amines react with the epoxy resin to form the polyurethane. The polyurethane is then cured as a result of the hardener's multifunctionality. Unfortunately, because all the more reactive amine groups have previously reacted, there is often a shortage of less reactive amine groups in the curing stage which leaves the reaction incomplete and weakens the structure of the final polyurethane.
In many epoxide resin-amine hardener formulations, reactions are carried out in the presence of organic solvents, which are volatile air pollutants and sometimes carcinogenic. These organic solvents also decrease the reactivity of the functional groups, thus reducing the degree of cross-linking.